It is a general requirement of audio circuits that that no audible ‘pops’ or ‘clicks’ occur through the loudspeaker or headphone when the circuit is powered up or powered down. Pops occurring during start-up or shutdown are generally caused by transient voltages across the load. Careful circuit design can avoid many start-up transients, for example by pre-charging capacitors to proper voltages. In Class D (PWM) amplifiers, the pulse train also can be modified at start up by a number of known techniques.
Even with such measures, a problem remains, however, particularly if there is a DC offset at the amplifier output during normal operation. When the amplifier turns on, the output voltage goes from zero volts to the steady-state offset voltage in a relatively short time period, resulting in a pop.
In a full-bridge type amplifier, or one with split rail (positive and negative) voltage supplies, there may be nominally no DC component at the output, but in reality some DC offset will always be present. To achieve low DC offset voltages, accurately matched components and circuits are required, which tends to increase their size, resulting in higher silicon area and associated cost. For this reason, an economic implementation of such an amplifier will always have some measurable offset, and thus possibly generate a pop on start-up.
The known measures also do not address a further source of transients that has been identified by the inventor. This arises in amplifiers where the quiescent input and output voltages (ground reference levels) are different by design, for example where the input stage and output stage operate at different supply voltages. Such amplifiers will be referred to herein as ‘dual-supply’ amplifiers for convenience, although the principles of the invention apply to amplifiers with different ground reference voltages in input and output stages, even if the supplies to each stage are nominally the same.
Where the input and output stages are DC coupled, a bias current or voltage can be applied somewhere in the circuit to ensure that the output is biased half-way between the output rails (or at whatever voltage is defined as the ground reference level for the output signal) when the input signal is biased half-way between the input stage supply rails (or at other defined input ground reference level). Typically the output will be pre-charged to its nominal quiescent bias voltage in a gradual fashion before fully activating the output amplifier, for example by pre-charging an output a.c. coupling capacitor from large-value resistor divider to give a long time constant. But any difference between this pre-charge voltage and the output voltage from the output amplifier derived with the bias signal will result in an associated startup transient. To reduce such a difference to, say, one part in a thousand of the full-scale audio signal requires that both the pre-charge voltage and the bias signal are accurate to a similar precision to this, despite for example inevitable random manufacturing mismatches between components in circuitry deriving them, second-order circuit effects, and the effect of tolerances in supply voltages. As an alternative to DC coupling and setting the quiescent output level via a bias current injected at the input, AC coupling via capacitors and setting the quiescent output level by local d.c. feedback around the output stage is also possible, but may not be attractive in an integrated circuit implementation. Furthermore, a local d.c. feedback arrangement also has its own start-up and offset problems.